RFM66W Datasheet, PDF (36/87 Pages) List of Unclassifed Manufacturers – 868 & 915MHz Ultra Low Power High Link Budget Integrated UHF Transceiver

WIRELESS & SENSING
RFM66W
DATASHEET
3.5.11. Preamble Detector
The Preamble Detector indicates the reception of a carrier modulated with a 0101...sequence. It is insensitive to the
frequency offset, as long as the receiver bandwidth is large enough. The size of detection can be programmed from 1 to 3
bytes with PreambleDetectorSize in RegPreambleDetect as defined in the next table.
Table 19 Preamble Detector Settings
PreambleDetectorSize
00
01
10
11
# of Bytes
1
2 (recommended)
3
reserved
For proper operation, PreambleDetectTol should be set to be set to 10 (0x0A), with a qualifying preamble size of 2 bytes.
PreambleDetect interrupt (either in RegIrqFlags1 or mapped to a specific DIO) goes high every time a valid preamble is
detected, assuming PreambleDetectorOn=1.
The preamble detector can also be used to ensure that AFC and AGC are performed on valid preamble. See Figure 9 for
details.
3.5.12. Image Rejection Mixer
The RFM66W embeds a state of the art Image Rejection Mixer (IRM). Its default rejection, with no calibration, is 35dB typ.
The IQ signals can be calibrated by an embedded source, pushing the image rejection to typically 48dB. This process is
fully automated and self-contained.
3.5.13. Image and RSSI Calibration
Calibration of the I and Q signal is required to improve the RSSI precision, as well as good Image Rejection performance.
On the RFM66W, IQ calibration is seamless and user-transparent. Calibration is launched:
 Automatically at Power On Reset or after a Manual Reset of the chip (refer to section 7.2). For applications where
the temperature remains stable, or if the Image Rejection is not a major concern, this one-shot calibration will suffice
 Automatically when a pre-defined temperature change is observed
 Upon User request, by setting bit ImageCalStart in RegImageCal
A selectable temperature change, set with TempThreshold (5, 10, 15 or 20°C), is detected and reported in TempChange, if
the temperature monitoring is turned On with TempMonitorOff=0.
This interrupt flag can be used by the application to launch a new Image Calibration at a convenient time if
AutoImageCalOn=0, or immediately when this temperature variation is detected, if AutoImageCalOn=1.
The calibration process takes approximately 10ms.
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